In recent years, the communication traffic volume has dramatically increased through progress of information technology, and it is expected that the communication traffic volume will further increase hereafter. In order to handle such an increase of the communication traffic volume, optical communication systems are required to further increase their communication capacity (transmission capacity). However, it is expected that conventional optical communication systems using single-mode fibers (SMFs) will reach the limit of their capacity. As a technology to go beyond the limit, research and development of spatial division multiplexing (SDM) are being actively conducted. An optical fiber for performing SDM includes a multi-core fiber (MCF), in which one optical fiber is provided with a plurality of cores, and information is transmitted through each core, thereby increasing the transmission capacity.
The following Patent Documents 1 to 4 disclose prior art for measuring characteristics of a multi-core fiber. To be specific, Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. 2016-99290) discloses a technology, in which images of interference fringes of interference light between object light passed through a multi-core fiber and reference light are obtained, and the waveguide mode of each core of the multi-core fiber is evaluated. Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. 2016-57297) discloses a technology, in which cores are connected to each other at each position of two ends of an optical fiber provided with a plurality of cores, and optical characteristics of the optical fiber are measured in a state where the plurality of cores are connected in series.
Patent Document 3 (Japanese Unexamined Patent Application, First Publication No. 2015-230263) discloses a technology, in which pulse light is led into an optical fiber-transmission path including two different reference fibers and a multi-core fiber, and the loss component depending on structural components of the multi-core fiber is evaluated from the result of determining the intensity of backscattered light from two ends of the optical fiber-transmission path and from the crosstalk of the multi-core fiber measured by any method. Patent Document 4 (Japanese Unexamined Patent Application, First Publication No. 2015-81779) discloses a technology, in which light is led into a plurality of cores provided in an optical waveguide (an optical fiber) all together through first end surfaces of the cores, the brightness of emission light is measured through imaging a plurality of images of a second end surface of each core, and the insertion loss of each core is determined.
A Non-Patent Document 1 (R. Okuno et al., “Cutoff Wavelength Measurement of Two Core Multi-core Fiber”, Proc. of 2013 18th OptoElectronics and Communications Conference held jointly with 2013 International Conference on Photonics in Switching (OECC/PS), Kyoto, Japan, MS1-3.) discloses a measurement method of the cut-off wavelength of a multi-core fiber including two cores, in which light is led into the cores, and light emitted from the two cores is received without distinction.
In a multi-core fiber, since information is transmitted through each of cores provided in one optical fiber as described above, measurement with respect to each of the cores may be needed according to a characteristic of the multi-core fiber to be measured. For example, with respect to the cut-off wavelength of the multi-core fiber, measurement for each core is needed.
As a method of measuring the cut-off wavelength of an optical fiber, the multi-mode excitation method, the bending method or the like is known. In the multi-mode excitation method, the cut-off wavelength is measured based on a ratio between transmitted light power of a multi-mode fiber (a reference fiber) serving as a reference and transmitted light power of an object optical fiber when the object optical fiber is multi-mode excited (excited under multi-mode operation). In the bending method, the cut-off wavelength is measured based on the ratio between values of transmitted light power of an object optical fiber when different bending diameters (for example, bending diameter of 280 mm×1 turn, and bending diameter of 280 mm×1 turn and bending diameter of 60 mm×1 turn, or the like) are applied to the object optical fiber while the object optical fiber is multi-mode excited. The multi-mode excitation of the object optical fiber is obtained by leading transmitted light of the multi-mode fiber into a core of the object optical fiber.
When the cut-off wavelength of each core of a multi-core fiber is measured using these methods, it is necessary to multi-mode excite each of cores provided in the multi-core fiber. To be specific, an operation, in which the above multi-mode fiber is coupled to only one core of the cores provided in the multi-core fiber, and the cut-off wavelength is measured by receiving light emitted from the core coupled with the multi-mode fiber, has to be repeated as many times as the number of cores provided in the multi-core fiber. In this way, in order to measure characteristics of each core of the multi-core fiber, complicated work may be required, and the measuring time may increase.
In addition, the above conditions do not occur only in a case where the cut-off wavelength of the multi-core fiber is measured, but may also occur in a case where other characteristics of the multi-core fiber are measured. The other characteristics of the multi-core fiber include, for example, loss of the multi-core fiber, polarization mode dispersion (PMD), effective core area (Aeff), mode field diameter (MFD) and the like.